Side illuminated multimode waveguide

ABSTRACT

A multimode waveguide having a small thickness has light coupled to the side of the waveguide in order to provide a multimode operation of the waveguide for providing a continuous pattern of totally internally reflected light which is utilized to reconstruct holographic images from a holographic emulsion placed on a surface of the waveguide. This waveguide structure for reconstructuring a hologram has the capability of providing highly efficient hologram reconstruction while using ordinary light sources and is able to use beam diameters greater than the thickness of the waveguide.

This is a division of application Ser. No. 07/841,576 filed on Feb. 26,1992, now U.S. Pat. No. 5,295,208.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is addressed to improved waveguide holograms andparticularly multimode waveguides having superior input light coupling.

2. Discussion of the Background

Waveguide holography offers many advantages when compared toconventional holograms. Waveguide holograms provide for the recordingand reconstructing of holographic images with lightwaves which propagatealong optical waveguides. When this is contrasted with conventionalholograms, a higher image-to-background contrast is obtained and ahigher global diffraction efficiency is obtained with low diffractionefficiency materials. Furthermore, waveguide holograms provide minimizedillumination space and obstruction free viewing.

A waveguide hologram is strictly defined as a hologram whose imagewavefront is reconstructed with a guided light from the waveguide. Thus,a waveguide hologram (WGH) consists of an input coupler 10, thewaveguide itself 20 and the holographic emulsion 30 as shown in FIG. 1.A source of light 15 is coupled into the waveguide and this waveguide isnormally a sheet of transparent material with two surfaces which arelocally parallel and optically polished. The refractive index of awaveguide must be higher than the index of the environment in order toachieve the principles of waveguide transportation.

The different types of waveguides are distinguished by the size of thedielectric which constitutes the waveguide and by the mode ofillumination. Prior art devices in the waveguide hologram field utilizeedge illumination in a single mode waveguide or in a multimodewaveguide. Single mode waveguides are used to couple integrated circuitswith optics in interconnected electronic packages. These single modewaveguides are very clean, however, they require an extremely preciseorientation of the input light source. That is, these type of WGHs arevery thin and the light must be coupled at the edge very carefully toprovide proper alignment.

Another type of waveguide is a multimode waveguide which involvesinternal reflections.

The prior art edge lit multimode waveguides suffer from problems withcoupling efficiencies and a requirement for input light direction.Furthermore, the matching of the size of the input light i.e. thediameter, is an important factor in these edge lit multimode waveguidesas they are in the edge lit single mode waveguides. The categories ofwaveguide based upon the width (w) of an incident beam of light, theoptical waveguide thickness (t) and the wavelength of the incident lightwave (λ) are shown in FIG. 2a-c.

A first category of thin film waveguide as shown in FIG. 2a has λ˜t<<w.This is a thin guided layer coated on a glass substrate of the type usedin integrated optics. The major drawback to this type of structure ofcourse is that it is difficult to achieve high coupling efficiency andwhite light coupling is impossible.

A second category, as shown in FIG. 2b, has a thick substrate waveguidewherein λ<<t˜w. Although light coupling in the edge is easy, such lightcoupling creates multiple discrete "bounces" at the waveguide surfacesand as a result, edge lighting can only provide discrete holograms.

In the third category of FIG. 2c there is a dielectric block whereint>>w>>λ. This type of structure allows white light to be edge-introducedand illuminates a hologram with nobounce, however, it is much too bulkyto be used and to be of interest in the field of waveguide holograms.

These types of edge lit multimode waveguides have the above associateddisadvantages and it is the purpose of the present invention to providean improved waveguide structure which eliminates these disadvantages andprovides ease of construction.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improvedconstruction for a waveguide hologram which is able to have increasedcoupling efficiency, total internal reflection and the ability to use awide variety of non-critical light sources. There is also an object ofthe present invention to form a spatially continuous pattern of totallyinternally reflected light in a multimode waveguide with a normalunmodified light source.

These and other objects of the present invention are obtained by awaveguide construction in a waveguide environment capable of supportingmultimode operation and in which the light source enters the side of thewaveguide instead of the edge of the waveguide.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a prior art edge lit wave guide hologram structure;

FIG. 2a-c shows various types of waveguide based upon the thickness ofthe dielectric material;

FIG. 3 details a multimode side illuminated waveguide hologram accordingto the present invention;

FIG. 4 illustrates multiple utilization of the illuminated beam of thewaveguide hologram of FIG. 3; and

FIG. 5 details the reconstruction of the recorded holographic image inthe system of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designateidentical or corresponding parts throughout the several views, and moreparticularly to FIG. 3 thereof, there is described the embodiment whichmeets the objects of the present invention and which provides a sideilluminated waveguide hologram.

The light source for the hologram of the FIG. 3 is attained by way of aninput coupler mechanism 12 which conducts light from a source into thewaveguide. This input coupler can be a prism or a grating or an opticalfiber. The waveguide itself 22 is a sheet of transparent material withtwo surfaces which are locally parallel and optically polished. Therefractive index of the waveguide must be higher than the index of theenvironment in order to achieve waveguiding. The wave which is coupledin is confined in the waveguide by total internal reflection on thewaveguide surfaces and propagates along a zigzag path as illustrated.The holograph emulsion 32 is placed parallel to and immediately incontact with the waveguide. This holographic recording material can be asilver halide emulsion, a photopolymer layer, a dicromated gelatin filmor a photoresist coating. When the hologram is illuminated with theguided wave, the previously recorded holographic image is reconstructed.

When compared with conventional holography, the waveguide hologramsprovide a compact system without requiring the kind of alignmentrequired for conventional holography. Because of the flexibility of theoptical fiber, the laser or incoherent source which is used can beremotely located. The waveguide hologram system is flat and it can behung on a wall or hand held without concern as to its illumination.Furthermore, the reconstructed image in a waveguide hologram isobstruction free and because the illumination beam is confined in thewaveguide it cannot be blocked. Because of the high image to backgroundcontrast and multiple utilization of the illumination beam as shown inthe FIG. 4, a bright image can be obtained. Furthermore because theimage can only be reconstructured by the light inside the waveguideother light sources will not affect the quality of the image.

The utilization of a multimode waveguide is illustrated in FIG. 3. Usingside illumination provides for improved coupling efficiency over edgeilluminated waveguides and further allows for use of an easily directedlight source without requiring modification of the light source. Withthe type of system shown in FIG. 3, although a laser could be utilized,either white light or other sources of light having a wide beam can beused. It is to be noted that in edge lit illumination systems, there isa restriction on the width of the input light beam. That is, the inputlight beam can be no larger than the thickness of the wave guide.

The side illuminated multimode waveguide is particular advantageous inconjunction with waveguides which have a thickness greater than thewavelength but yet the thickness can be less than the width of anincident light beam i.e. λ<<t<w. Because the thickness is much greaterthan the wavelength, the difficulties of thin film waveguides areovercome and white light illumination can be conveniently used.Furthermore, because the thickness is less than the width of the beam,uniform illumination is obtained. These advantages are brought out bythe side illumination input light coupling of FIG. 3 and provide for asignificant ease of construction and a compact package. The utilizationof a waveguide with the thickness much greater than the wavelength butless than the width of the input beam allows for use of a side inputcoupled light source with a relatively wide beam width in order to forma spatially continuous pattern of totally internally reflected light.This multimode side input coupled waveguide hologram provides for amultiple utilization of the illumination beam as shown in FIG. 4 andfunctions to provide an undiffracted beam, confined in the waveguide, toreconstruct the holographic image as shown in FIG. 5.

The FIG. 4 illustrates the illumination process wherein the collimatedguided illumination beam, when it reaches an area where the hologram isplaced, first encounters the region 1 of the hologram. A part of thelight is diffracted as the reconstruction of the image and the rest ofthe light is reflected. After total internal reflection at the otherwaveguide surface, the residual light illuminates the region 2 on thehologram and undergoes the second reconstruction. This process isrepeated until the illumination beam passes the hologram. Because of themultiple utilization of the illumination beam, the holographic imageconstructed by the FIG. 3 embodiment is more efficient than inconventional holography. The portions of the beam that are undiffractedremain confined in the waveguide and therefore the undiffracted lightmakes no contribution to the background brightness. Thus, a bright imagecan be obtained even with an inefficient hologram by simply increasingthe power of the illumination beam. This increased power will increasethe brightness of the image with no contribution to the backgroundbrightness because, as indicated above, the undiffracted light confinedwithin the waveguide makes no contribution to the background brightness.

A significant factor in the improvement of performance and simplicity ofconstruction is the use of the side input coupled light in contrast toedge lit structures. The edge lit waveguide structures require either alaser beam or a thick wave guide in order to function properly. With thepresent structure using a side light input coupling, full internalreflection is obtained with ordinary light sources, including fiberoptic input which allows for remote non-critical location of the actuallight source.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

What is claimed as new and desired to be secured by letters patent ofthe United States is:
 1. A waveguide hologram for illumination with alight beam having at least one spectral component with a wavelength λ,said waveguide hologram comprising:a waveguide for supporting multimodepropagation of light, and having first and second main surfaces defininga thickness dimension disposed therebetween, a widthwise extent havingopposing ends, a lengthwise extent having opposing ends, and first andsecond edge surfaces disposed at the opposing ends of said lengthwiseextent; input coupling means, disposed at one of said first and secondmain surfaces in the vicinity of said first edge surface, for couplingsaid light beam into said waveguide so that said coupled light beamprovides a spatially continuous pattern of light within said waveguideand the rays of said spatially continuous pattern of light are totallyinternally reflected therewithin, and the thickness dimension of saidwaveguide being greater than the wavelength λ of said at least onespectral component of said light beam coupled into said waveguide; and arecording medium disposed on one of said first and second main surfacesand containing a recorded hologram consisting of an interferencepattern.
 2. The waveguide hologram according to claim 1, which furthercomprises light beam producing means for producing said light beam. 3.The waveguide hologram according to claim 2, wherein said light beamproducing means comprises a source of coherent light.
 4. The waveguidehologram according to claim 2, wherein said light beam producing meanscomprises a source of non-coherent light.
 5. The waveguide hologramaccording to claim 3, wherein said light beam producing means compriseslaser.
 6. The waveguide hologram according to claim 1, wherein saidinput coupling means is a grating or a prism.
 7. The waveguide hologramaccording to claim 1, which further comprises a light beam producingmeans for producing a point source of light which is coupled to one ofsaid first and second main surfaces by way of said input coupling means.8. The waveguide hologram according to claim 6, which further compriseslight beam producing means for producing a white light beam that iscoupled to said first or second main surfaces.
 9. The wavelengthhologram according to claim 1, wherein said continuous pattern of lightinteracts with said recording medium in such a way that a portion ofsaid light beam is diffracted, while the remainder of the light beam isreflected from said first or second main surface supporting saidrecording medium.
 10. The waveguide hologram according to claim 1,wherein the thickness dimension of said waveguide is less than the widthof said light beam coupled to one of said first and second main sidesurfaces.
 11. The waveguide hologram according to claim 1, wherein saidinput coupling means comprises one or more optical fibers.
 12. Thewaveguide hologram according to claim 1, wherein said waveguidecomprises an optically transparent sheet.
 13. The waveguide hologramaccording to claim 1, wherein said first and second main surfaces aresubstantially parallel.
 14. The waveguide hologram according to claim 1,wherein said interference pattern contains information of 3-D object.